This invention relates to acoustic emission testing of materials and structures and more particularly to detection and measurement of crack growth in noisy environments.
Acoustic emission is the term given to stress waves created in materials by the sudden release of energy resulting from irreversible processes such as crack growth, plastic deformation, and phase transformations in solid materials. Acoustic emission (AE) techniques have been used in the field for over 25 years for the nondestructive (NDE) testing of structures, including metal and composite pressure vessels and piping. It has also found wide application in the testing of composite man-lift booms. Currently, AE techniques are used primarily for locating cracks and potential problem areas in metal structures for pressure boundary applications, while other types of nondestructive techniques are necessary to provide acceptance or rejection criteria. The AE technology has not achieved the same level of acceptance as other nondestructive techniques for the field testing of structures such as bridges and other components of infrastructures for two primary reasons: (1) the difficulty in separating valid signals from those caused by extraneous noise, and (2) the inability of the AE techniques to determine the size of the crack or flaw.
When a crack grows in a metallic material under stress due to fatigue, stress corrosion cracking, or hydrogen embrittlement cracking, a small amount of strain energy is released in the form of a stress wave that propagates from the source of the crack at the velocity of sound in the material. The increment of time in which the strain release occurs is on the order of a microsecond or less. Thus the frequency content of the stress wave is very broad band, ranging from a few kilocycles to over 1 megacycle in frequency. In solids, two types of stress waves can exist in the bulk of the material: an extension wave, where the particle motion is parallel to the direction of propagation, and a shear wave, where the particle motion is perpendicular to the direction of propagation.
The energy of the stress wave from the crack usually consists of both an extension wave and a shear wave. Most structures constructed from metal are plate-like in nature, examples of which include pressure vessels, bridges, and aircraft. Therefore, the stress wave shortly after initiation will strike a boundary. If it strikes the boundary at an angle, Snells Law prevails and mode conversion of an extensional wave to a shear wave can occur, while a shear wave can mode-convert to an extension wave. The behavior of the stress wave can become very complicated by the time it has traveled a distance of several plate thicknesses away from the crack. In this situation, the propagating waves will be governed by Lamb""s homogeneous equation, the solution to which are known as Lamb waves. In the limit where the wavelength is much larger than the plate thickness, a simpler set of governing equations derived from classical plate theory can be used to model the motion. Under classical plate theory, the waves are called plate waves, and there are two modes of propagation, namely, the extensional mode and the flexural mode. Both have in-plane (IP) motion and out-of-plane (OOP) motion. The OOP motion is the greatest for initial displacements of a source perpendicular to the plane of the plate, and the IP motion is greatest for initial displacements of a source parallel to the plane of the plate. For example, the sudden propagation of a crack will create primarily an IP wave, because the crack normally grows in a direction perpendicular to the plane of the plate, while impacts on the surface of the plate will create primarily OOP sources, since the initial source function creates a bending or flexural wave.
The stress waves (acoustic emission (AE) events) in present practice are detected by a piezoelectric transducer that is attached to the surface of the structure with vacuum grease, vaseline, or other couplants to provide an air-free path for the high frequency waves to reach the active element of the transducer. The transducer used in the majority of the tests has a resonant frequency of approximately 150 kHz. When the AE waves strike the transducer, they set it xe2x80x9cringingxe2x80x9d at its resonant frequency. The use of a resonant transducer increases the sensitivity of detecting the AE events. Since the frequency contents of the waves are very broad band, they will activate the resonant frequency of any transducer having a resonant frequency between 20 kHz and 1 MHz. Most AE data is taken in the 100 to 500 kHz frequency range, where the data is low enough in frequency that attenuation effects are minimal, and high enough in frequency so that low frequency air borne noise is eliminated.
A majority of the practical AE tests are conducted on structures made from plates or plate-like components. Recent research has shown that out-of-plane (OOP) AE sources produce strong flexural wave components in a plate, with weak extensional components, while in-plane (IP) AE sources produce strong extensional waves in a plate with weak displacement components normal to the plate surface.
For many years, researchers and field test engineers employing acoustic emission techniques have used the breaking of a pencil lead on the surface of a structure or specimen to simulate the type of AE signal present when a crack propagates or when fibers break in composite structures. Because this is an OOP source, most of the energy goes into the flexural wave which is inherently low frequency, and only a small portion of the energy is carried by an extensional wave.
Michael R. Gorman, in his paper entitled xe2x80x9cPlate Wave Acoustic Emission,xe2x80x9d Journal of Acoustic Society of America, 90(1), July 1991, used broad band sensors to detect both types of waves. By mounting the transducer on the surface of an aluminum plate, extensional waves were simulated by breaking pencil leads on the edge of the plate, and flexural waves were simulated by breaking the pencil lead on the surface of the plate. Further work by Gorman and Prosser, xe2x80x9cAE Source Orientation by Plate Wave Analysis,xe2x80x9d Journal of Acoustic Emission, Vol. 9, No. 4 (1990), consisted of machining slots at different angles in a plate to observe the response when pencil leads are broken at an angle, which is measured from the plane of the plate. As expected, it was found that for 0 degrees, the highest signal amplitudes occurred for extensional waves, and for 90 degrees, the highest signal amplitudes occurred for flexural waves. A mixture of both waves was found for intermediate angles.
The broad band transducer used by Gorman and Prosser is problematic for a number of reasons. First, it was designed for ultrasonic testing with a resonant frequency of 3.5 MHz, and was presumed to have a flat frequency response from a few kHz to 1 MHz. However, although the measured frequency response of similar transducers yields a fairly flat frequency response from 300 kHz to 1 MKz, it is far from flat from 10 kHz to 300 kHz. In addition, its sensitivity in the frequency range below 1 MHz is an order of magnitude lower than those of resonant transducers normally used for acoustic emission testing. Consequently, the sensor must be placed close to the source in order to obtain good results. Moreover, when IP and OOP amplitudes are compared from data obtained by the transducer mounted on the surface, there is a large difference in the peak amplitudes measured from the different sources. Further, the procedure presently employed by Gorman cannot be used for crack growth measurement. Gorman merely discloses a method to digitize all signals and to attempt through visual examination and pattern recognition software to determine the amount of IP and OOP components present to make a decision regarding whether or not a signal is primarily one or the other type. Because the overwhelming number of AE signals detected in the field are extraneous noise (OOP), Gorman""s method requires an enormous amount of storage for the digitized signals.
One of the major reasons AE techniques have not been widely accepted is that the techniques will not give any quantitative information concerning the size of a crack or the amount of crack growth. Currently, AE is primarily used to locate the crack by the use of multiple channels. By measuring the time that each transducer receives the AE signal, and ascertaining the velocity of sound in the material, the source location can be calculated.
The second main reason for the lack of wide application of AE techniques to monitor structures in the field is the difficulty in separating extraneous background noise from the AE signals coming from the crack. Impacts on the field structure from wind-blown sand, particles, rain, maintenance personnel, and leaks in pressurized components all can give noise signals in the frequency band of interest. Rubbing friction between components is another source of extraneous noise that has frequency components in the frequency range of interest. Most extraneous noise sources of this type are out-of-plane (OOP) sources, and although they can have very high-frequency components in an undamped structure, most of the energy in the stress waves created by such sources in most structures constructed from plates can be found at frequencies below 100 kHz. This energy is carried by a low-frequency flexural wave in the plate. AE signals generated by crack growth, on the other hand, are in-plane (IP) sources, and most of the energy in the stress wave is carried by high-frequency extensional and shear waves.
There is a need, therefore, for a method and an apparatus of eliminating extraneous noise in an AE signal and determining crack size from the AE signal.
This invention provides a solution to the above major problems encountered in current acoustic emission testing by using the ratio of the extensional wave to the flexural wave as a filter for eliminating extraneous noise and determining crack size. Special transducer and instrumentation techniques are used to recognize the type of wave predominant in a plate or plate-like structure to allow filters to be constructed in the instrumentation to not only eliminate extraneous noise sources from the AE data, but also make available nondestructive measurement of the depth of a growing crack in the structure by AE techniques. In addition, OOP signals are eliminated early on so that a smaller amount of storage is required to store the data for crack-like IP signals. The invention will be discussed in relation to crack growth in metals, but is not restricted to metals only.
In accordance with one aspect of the present invention, a false aperture transducer is formed by partially mass-loading a large crystal with a mass. The partially mass-loaded portion of the crystal reacts as a displacement-sensitive element and responds well to out-of-plane (OOP) sources, while the remainder of the crystal is sensitive to the higher frequency in-plane (IP) sources. The ratio of sensitivity to the high-frequency IP source and low-frequency OOP source can be calibrated to a specific value. The presence of an OOP signal will produce a ratio of the high-to-low frequency amplitude of lower than the sensitivity ratio of the transducer and an IP signal will produce a frequency amplitude ratio higher than the sensitivity ratio. In a preferred embodiment, the sensitivity ratio of the transducer is calibrated to be equal to one so that the transducer is equally sensitive to OOP and IP sources.
To obtain the high-to-low frequency amplitude ratio of an AE signal, a false aperture transducer is used to sense the signal. The AE signal is split into a high-frequency component and low-frequency component with analog filters and amplified. The high-frequency component is filtered with a high-pass filter, and the low-frequency component is filtered with a bandpass filter. The peak amplitudes of the two components are measured, from which the frequency amplitude ratio is calculated.
The high-to-low frequency amplitude ratio can be used to filter out extraneous noise by eliminating ratios that are lower than the calibrated sensitivity ratio of IP to OOP sources of the false aperture transducer. A portable instrument can produce an audio signal to an operator upon detecting a ratio higher than the sensitivity ratio, which indicates a crack-like IP source. Location of the crack-like source can be determined by detecting the AE signal from three different locations and ascertaining the point of intersection.
Significantly, the determination of crack size is also made possible by computing the high-to-low frequency amplitude ratio. To do so, a calibration curve correlating crack depth with the frequency amplitude ratio is obtained by cloning a fracture specimen or similar block of material onto the structure to be tested or a plate-like specimen with similar thickness. Crack growth is simulated in the fracture specimen which is attached to the structure. A false aperture transducer on the structure receives the AE signal generated and the high-to-low frequency amplitude ratio is computed. By simulating different crack depths in the fracture specimen and computing corresponding amplitude ratios from the AE signal, a calibration curve is obtained. To measure crack depth, the false aperture transducer is placed on the structure and AE signals sent to an instrumentation for computing the high-to-low frequency amplitude ratio. The corresponding crack depth can be determined with the amplitude ratio by the calibration curve. The crack profile is approximated by assuming a semi-elliptical shape.